Understanding States of Matter and Phase Changes

1. Chapter 13 Changes of state

2. Phase changes Section 13.4

3. States of Matter Comparison of Gases, Liquids and Solids • Gases are compressible fluids. Their molecules are widely separated. • Liquids are relatively incompressible fluids. Their molecules are more tightly packed. • Solids are nearly incompressible and rigid. Their molecules or ions are in close contact and do not move. • Vapors term customarily used for the gasesous state of a substance that exists naturally as a solid or liquid at 25 C and 1 atmosphere

4. Energy Requirements for Phase Changes Water (solid) water (liquid) = 6 kJ/mol Water (liquid) water (gas) = 41 kJ/mol What does this mean???? Takes more energy to convert water from a liquid to a gas then to convert water from a solid to a liquid. WHY?? Solid and liquid states of water are more similar then the liquid and gas states

5. Section 13.1 Water and its phase changes FYI!!! water is unique in that as it cools it EXPANDS WHY??? Explain why ice floats? Why pipes burst in the Winter? How potholes form?

6. Phase Transitions H2O(s)  H2O(l) H2O(l)  H2O(s) H2O(l)  H2O(g) H2O(s)  H2O(g) H2O(g)  H2O(l) H2O(g)  H2O(s) • Melting: change of a solid to a liquid. • Freezing: change a liquid to a solid. • Vaporization: change of a solid or liquid to a gas. Change of solid to vapor often called sublimation. • Condensation: change of a gas to a liquid or solid. Change of a gas to a solid often called deposition.

7. Energy of Heat and Phase Change • Temperature does not change during the change from one phase to another. • notice: there is either a temperature change OR a phase change. • You cannot have BOTH at the same time • Increase in temperature = increase in kinetic energy (after all the definition of temperature is average kinetic energy). There is no change in the potential energy • Increase in heat = increase in potential energy. There is no change in the kinetic energy because the temperature does not change!!!!

8. Vaporization vs Boiling vs Evaporation • Boiling For water to boil, it must be 100 Celsuis. Boiling creates an actual gas. The substance changes phase.Evaporation also involves liquids become gaseous. However, the body of liquid does not need to be at the boiling temperature. It occurs because the molecules of a liquid are not tightly bound together, and so some escape with time.Vaporization is a blanket term referring to both boiling and evaporation. In the broadest sense, it is liquid becoming gas.

9. Boiling point vs vapor pressure • Boiling point the temperature at which the vapor pressure of a liquid is equal to the pressure of the external atmosphere. • Normal boiling point the temperature at which the vapor pressure of a liquid is equal to atmospheric pressure (1 atm). • Vapor Pressure the pressure of the vapor over a liquid at equilibrium in a closed container

10. Vapor Pressure • If a liquid is placed in a non-closed container, some of the molecules will escape the surface and evaporate. • In a sealed container, some of a liquid still evaporates but cannot “leave” the container. The molecules move back and forth between liquid and gas phases until they establish an equilibrium and thus a pressure in the vapor phase. • Vapor pressure: partial pressure of the vapor over the liquid measured at equilibrium and at some temperature.

11. Temperature Dependence of Vapor Pressures • The vapor pressure above the liquid varies exponentially with changes in the temperature. • What does this graph indicate about the relationship between vapor pressure and temperature?? • There is a _______ relationship between vapor pressure and temperature. In other words as vapor goes UP the temperature goes _______ or as vapor pressure goes DOWN the temperature goes _______

12. Phase Diagrams Section 13.4

13. Phase Diagrams • Graph of pressure-temperature relationship; describes when 1,2,3 or more phases are present and/or in equilibrium with each other. • Lines indicate equilibrium state two phases. • Triple point- Temp. and press. where all three phases co-exist in equilibrium. • Critical temp.- Temp. where substance must always be gas, no matter what pressure. • Critical pressure- vapor pressure at critical temp. • Critical point- point where system is at its critical pressure and temp.

15. Phase Diagram

17. Phase Diagram of Water

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